1. Field of the Invention
The present invention relates to a laser processing method of performing laser processing along a process line to the inside of a workpiece such as a semiconductor wafer and an optical device wafer.
2. Description of the Related Art
As well known in the art, in a semiconductor device fabrication process, a functional layer composed of an insulating film and a functional film is formed on the front side of a substrate such as a silicon substrate, and a plurality of semiconductor devices such as ICs and LSIs are formed like a matrix from this functional layer, thus obtaining a semiconductor wafer having the plural semiconductor devices. The plural semiconductor devices are partitioned by a plurality of crossing process lines called streets formed on the front side of the semiconductor wafer. The semiconductor wafer is divided along these streets to obtain the individual semiconductor devices.
Further, in an optical device fabrication process, an optical device wafer is provided by forming an optical device layer composed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer on the front side of a sapphire substrate or a silicon carbide substrate. The optical device layer is partitioned by a plurality of crossing streets to define a plurality of regions where a plurality of optical devices such as light emitting diodes and laser diodes are respectively formed. The optical device wafer is cut along the streets to thereby divide the regions where the optical devices are formed from each other, thus obtaining the individual optical devices.
As a method of dividing a workpiece such as a semiconductor wafer and an optical wafer along the streets, there has been tried a laser processing method of applying a pulsed laser beam having a transmission wavelength to the workpiece along the streets in the condition where the focal point of the pulsed laser beam is set inside the workpiece in a subject area to be divided. More specifically, this workpiece dividing method using laser processing includes the steps of applying a pulsed laser beam having a transmission wavelength to the workpiece from one side of the workpiece along the streets in the condition where the focal point of the pulsed laser beam is set inside the workpiece to thereby continuously form a modified layer inside the workpiece along each street and next applying an external force to the workpiece along each street where the modified layer is formed to be reduced in strength, thereby dividing the workpiece into the individual devices (see Japanese Patent No. 3408805, for example).
As another method of dividing a workpiece such as a semiconductor wafer and an optical device wafer along the streets, there has been put into practical use a technique including the steps of applying a pulsed laser beam having an absorption wavelength to the workpiece along the streets to thereby form a laser processed groove along each street by ablation and next applying an external force to the workpiece along each street where the laser processed groove is formed as a break start point, thereby breaking the workpiece along each street (see Japanese Patent Laid-open No. Hei 10-305420, for example).